The present invention pertains to device fixtures and, more particularly, to cooled device fixtures.
Many semiconductor devices, such as, for example, radio frequency (RF) semiconductor devices, are manufactured in factories including equipment for individually testing the electrical performance of each device. One such test is commonly referred to as a burnout test in which heat dissipation and thermal conditions under which the device is operated may be extreme and may lead to device failure if the device is not adequately cooled during testing. Because devices that are individually tested tend to sell for relatively high prices, any yield degradation caused by testing directly impacts the profit of the company. For example, for every $100 RF device damaged at test, the company will not realize the $100 of revenue from the sale of that device.
Presently, when devices are individually tested, each device is placed in a specially designed cooling fixture including a conduction-cooled heat sink that may have an associated fan. After the device has been placed on the cooling fixture, it is clamped into place to prevent movement of the device and to allow the device under test to conduct heat to the cooling fixture. It is not uncommon for the device to be clamped into the cooling fixture with a clamp force of as much as 30 pounds (lbs.), which can lead to unintended damage of potentially fragile parts or structures inside the device. Additionally, the 30 lb. force can be unwieldy and difficult to control.
Because the device and the test fixture are not perfectly planar, there exists a small gap between the bottom face of the device being tested and the top of the cooling fixture when the device is placed on the cooling fixture. For example, the gap may be due to surface roughness and features on each of the mating interfaces. The air gap between the device under test and the cooling fixture inhibits thermal conduction between the device and the fixture, thereby preventing the device from easily coupling its heat to the cooling fixture and resulting in device heating that may result in increased device die temperature. Accordingly, to enhance the thermal conduction path between the device and the cooling fixture, a thin layer of thermally conductive grease such as, for example, Wakefield grease is commonly applied to the contact surface of the device before the device is clamped into place on the cooling fixture. Such grease is a non-water soluble thermal conductor. While the Wakefield grease aids in thermal conduction, grease thickness and air pockets in the grease may lead to inconsistent or unpredictable thermal conduction during device testing.
After testing of the device is complete, the clamp holding the device to the fixture is released and the device is manually removed from the fixture using equipment such as tweezers. An operator then uses cotton swabs and a methanol based solvent to remove the grease from the device that has been tested and the device is placed into a sorting bin representative of the electrical characteristics of the device. Care must be taken to ensure that all grease residue is removed from devices because, once purchased, devices are commonly soldered into place as parts of systems or subsystems. Failure to remove absolutely all of the Wakefield grease residue from the device would contaminate the soldering process, thereby yielding cold solder joints, poor bonding and potentially open circuits. In practice, however, some of the grease residue will always remain on the device. Whether such residue affects manufacturing processes depends on the quantity of residue.
As will be readily appreciated, the foregoing process requires manual labor to apply the Wakefield grease to the device to be tested and to remove the grease from the device after testing is complete. Because certain devices are 100 percent tested (i.e., each device leaving the factory is tested) the manual labor costs associated with device testing could be considerable. In fact, while electrical testing of devices may require on the order of 50 seconds of testing time, the manual labor associated with applying the Wakefield grease to the device before testing and removing the same from the device after testing may equal the test time, thereby doubling the process time for testing a device. Accordingly, not only is the use of the Wakefield grease expensive in terms of manual labor costs, it is expensive in terms of product throughput time. Furthermore, some grease residue will always remain on the device, which could affect the processing of the device by the purchaser.